Permselective devices have proven useful to selectively remove or replace certain substances (species of interest) in a solution. Current use of permselective devices to both recover valuable products and remove harmful materials is widespread in many fields including clinical medicine, analytical and clinical chemistry, and the electrochemical and chemical process industries.
These devices utilize a barrier comprised of a permselective material, typically a membrane, which allows facile permeation, or transport, of certain particles, molecules or ions through the barrier while offering large resistance to the transport of other particles, molecules or ions. The species of interest passes through the membrane barrier at numerous transport sites located on the membrane. Thus, the number of available transport sites on a given membrane barrier may be a limiting factor in the overall rate of transport. This is determined by the type as well as the dimensions of the membrane used in the device. Previously known permselective membranes include cation exchange and anion exchange membranes.
Cation exchange membranes electrostatically resist the passage of anions while permitting the passage of cations at cation exchange sites located on the membrane. Nafion, a commercial cation exchange membrane of E. I. du Pont de Nemours & Co., comprised of a fluorocarbon backbone with pendant sulfonic acid groups as ion exchange sites, has proven a useful membrane for use in the field of ion chromatography.
Anion exchange membranes, on the other hand, electrostatically resist the passage of cations while permitting the passage of anions at anion exchange sites located on the membrane. RAIPORE-ADM, a commercial anion membrane of RAI Research Corp. comprised of radiation-grafted polyethylene-based anion exchanger, has also proven a useful membrane in ionic separation procedures.
Permselective membranes are commonly configured as flat sheet membranes, as in most electrochemical applications, or as hollow fibers, as in haemodialysis devices. In operation, tubular permselective membranes are commonly enclosed within an external jacket, thus creating an assembly having two channels through which solutions may pass: a central channel within the tubular membrane itself and an outer channel between the membrane and the external jacket. Typically, a solution containing the species of interest is passed through the central channel of the membrane tube and a regenerant solution is passed through the outer channel. The species of interest makes contact with the inner surface of the membrane tube and diffuses through the membrane. A regenerant solution containing a species capable of rejuvenating exhausted membrane transport sites is passed through the outer channel. This not only facilitates recovery of the species of interest after it has been removed from its original solution, but also enables the membrane to further transport the species of interest as more solution containing the species of interest passes through the central channel and makes contact with the inner surface of the membrane tube. The flow of solutions through these channels may be accomplished by gravity alone or facilitated with the aid of a pump. In this tubular configuration, the overall efficiency of transport of the species of interest is maximized by minimizing the rate at which the species of interest flows through the central channel and, to a degree, maximizing the rate at which the regenerant solution flows through the outer channel. Thus, the flow rates of both solutions are limiting factors in the overall rate at which transport of the species of interest across the permselective barrier takes place.
The overall efficiency is also dependent, in part, upon the rate of transport of the species of interest through the membrane and the rate of transport to the membrane. The transport rate through the membrane is dependent upon the nature of the membrane and the thickness of the membrane. For any given membrane type, structural considerations limit the degree to which the thickness of the membrane can be reduced. Although thinner membranes facilitate mass transfer through the barrier, they are, however, less able to withstand pressure and run the risk of rupture during operation. Further, the resistivity of all membranes towards the undesirable species (i.e., those which normally should not penetrate the membrane) also decreases with decreasing membrane thickness. Thus, the use of thinner membranes lead to additional considerations in the use of regenerants. For example, penetration of regenerants becomes significant at relatively high regenerant concentrations and with thinner membranes. Regenerant penetration is an even greater threat where the rejuvenating species is of low molecular weight, such as sulfuric acid or sodium hydroxide.
The rate of transport through the membrane is related to the ratio of the number of membrane sites available for transport to the flux of the species of interest. Thus, for a given membrane and a given number of available transport sites, the overall rate of removal is likely to be limited by the rate of mass transport through the membrane where the flux of the species of interest is high. However, where the flux of the species of interest is low, the overall rate of removal is likely to be limited by the rate of transport to the membrane.
In the tubular configuration, the hold up volume, or dead space within the central channel, can be the most significant limiting factor in the rate of transport to the membrane. For a given length of the hollow tubular device, the hold up volume is proportional to the square of the diameter. An increase in the hold up volume decreases the efficiency of transport to the membrane. Although an increase in the length of the tube provides a greater amount of surface area for transport, it is desirable to keep the length to a minimum to avoid increased pressure drops.
The hold up volume must often be kept to an operational minimum for reasons other than mere efficiency of transport. The small hold up volume requirement in ion chromatography arises from the need to minimize the dispersion of an injected sample, to maintain the resolution (minimize overlap between two adjacent sample bands) and sensitivity (minimize dilution of the injected sample).
Since the hold up volume is dependent upon the length and inner diameter of the tube, the overall process is optimized by using membrane tubes of the shortest possible length and the smallest possible inner diameter. However, 100 microns is the current technological lower limit for the inner diameters of hollow membrane tubes. In fact, many commercial membranes either are not or cannot be commercially manufactured even at this diameter.
Previous work has been directed toward improving mass transfer to the walls of a tube by decreasing its hold up volume. One approach has accomplished this objective by filling the tube with inert beads of optimized size as described in Reijn, J. M., et al., Anal. Chim. Acta, 123:229 (1981). These beads act to reduce the hold up volume within a tube and thus increase the transport efficiency to the wall per unit hold up volume. Another approach directed toward decreasing the hold up volume was discussed and disclosed in my copending U.S. patent application Ser. No. 421,082, filed Sept. 22, 1982, now U.S. Pat. No. 4,500,430, and in Dasgupta, P. K., Anal. Chem. 56:96-103 (1984). This approach requires inserting a closely fitting filament inside the tubular membrane such that the liquid flows in the annular space between the filament and the membrane. The filament functions to occupy space within the tube and thus decreases the hold up volume. In the preferred embodiment, the filament filled tube is coiled into a helical configuration having a small helical diameter. The process is most efficient where the diameter of the helix is kept to a minimum.
Both of the above-described approaches greatly reduce the hold up volume of the tubular membrane device compared to its hollow configuration. The filament filled helical configuration has certain additional advantages as described in my copending U.S. application Ser. No. 421,082 and has proven to be a valuable technique to conduct efficient mass transfer to the membrane.
Despite the advancements previously made, there is still a need for an improved permselective device having minimal hold up volume and an increased surface area for transport, particularly to deal with situations where mass transfer through the membrane is the limiting factor. It is thus an object of the present invention to increase the overall rate of transport in a tubular permselective device.
It is a further object of the present invention to provide a perselective device capable of simultaneously removing more than one type of species of interest from a sample.
It is yet another object of the present invention to provide a permselective device capable of simultaneously removing at least one species of interest from two samples.